I think it starts with two pairs from different parents (same breed) and they used standard white Cornish paired with white Plymouth Rocks and alternated the male from one breed to female on the second pair. AB and CD and then bred the babies together to get the Cornish hybrid. I saw a chart online, but can't remember where...
I am experimenting with Bresse/Cuckoo Lemon Orpington and Australorp/Bresse. I want to see how their offsprings turn out. The Bresse chickens mature faster than other heritage breeds, similar to Plymouth rocks. My Lemon Orpington line grows big (10 to 13lbs), but they take long to grow. My Australorp line produces a lot of eggs, but they seem smaller.
My Bresse roosters come from different farms and they look different. One has shades of tan high light on white and the other is white with black bleeding through. I used my black Australorp hens with the one with black bleeding through to see if I can bring back the original pencil color Bresse. Also the rooster with black bleeding through had a black dot on yellow fluff as a chick, so I may be able to get a sex link.
I also have a couple Jersey Giant/Bresse eggs in my incubator. Although, I put my breeders back together, I could tell which egg came from my single Jersey Giant hen, her eggs are the biggest and she is the heaviest of all the hens, but she don't lay as much eggs as the Orpington and Australorp.